The operation of corrugator machines and of corrugated board production lines, and the problems of warp generation resulting in the corrugated board during its manufacture are well known in the art. Board warp, or cupping, in corrugated board is caused by a moisture imbalance between the top liner board and bottom liner board of the combined corrugated board. As is known, the tips of a fluted web medium are glued to the top liner board forming a single face web. Downstream along the corrugated board production line, glue is applied to the exposed tips of the fluted web of the single face board to glue the single face board to the bottom liner board, also known as the double backer, thus forming the combined corrugated board.
The corrugating process is primarily a drying process involving the application of a water based adhesive glue, know within the corrugation board industry as starch, to the fluted web when gluing the web to the top liner or single face board, and then later applying glue to the bottom liner or double backer board to form the combined board. Heat is applied to the various components of the corrugated board at different steps in the process either to cure or dry the water in the starch mixture for causing the starch to bond to the paper surface of the top and bottom liner boards, respectively, resulting in a laminated structure which is rigid and from which strong boxes, for example, can be fabricated. However, when a difference in moisture content between the top liner board and bottom liner board in the combined board is present, which inevitably occurs during the drying process, the liner board which has a lower moisture content will shrink at a greater percentage rate than the liner board with the greater moisture content, resulting in a cup or warp in the combined board toward the relatively dryer liner board.
Thus, in an effort to address the cupping or warping of corrugated board during its manufacture, attempts have been made to provide devices which measure and control the temperature of the corrugated board during the manufacturing process in the attempt to control moisture content of the corrugated board components. Temperature, as is moisture content, is an important variable in the corrugating process, especially as it relates to starch formation and curing, and thus temperature has been more commonly used than moisture content measurement as a control factor in the production of corrugated board.
One example of the prior art is disclosed in U.S. Pat. No. 3,004,880 to Lord, issued Oct. 17, 1961. Lord discloses a control method and apparatus in which four pyrometers measure the temperature of the top liner board, the web medium, which later becomes the fluted web, the single face web, and the bottom liner board. Each pyrometer transmits an electrical signal proportional to each of the respective temperature conditions to an associated controller. Thereafter, when a production line operator visually observes cupping in the corrugated board, the operator will manually adjust any one or combination of the controllers to which the pyrometers provide data, the controllers in turn controlling the movement of the wrap arms associated with the cylindrical heating drums of conventional wrap arm heater assemblies stationed along the corrugated board production line to increase or decrease the temperature of the corrugated board.
In Krayenhagen et al., U.S. Pat. No. 5,244,518, issued Sep. 14, 1993, a corrugated board manufacturing apparatus and process is disclosed which includes steam dryer manifolds for zone drying of the web using pressurized steam to produce moisture to equalize the moisture content across the width of the corrugated board with respect to its length, the apparatus including moisture sensors and temperature sensors which transversely scan the web and provide feedback information regarding web moisture and temperature. Although Krayenhagen et al. measure moisture content across the width of the top liner board and bottom liner board, this information is used only to increase or decrease the amount of steam passed through steam manifolds provided to increase or decrease moisture content in the top liner board and bottom liner board, respectively.
Variations in moisture content of the components of the corrugated board arise for several reasons. The amount of water in the starch mixture can vary, so that the percentage of solids in the starch to water forming the glue mix is not uniform in the glue applied to the web medium as it is first glued to the top liner board, and later glued at a separate station to the bottom liner board. In addition, the amount of starch applied to the top and bottom liner boards can vary, which results in a variation of moisture being added to the liner board. Also, the moisture content in the top liner board, medium board, and bottom liner board can and does vary from board to board. This is largely due to the fact that each roll of board paper is approximately 6 to 7 ft. in diameter and will be several thousand feet long. Thus, each roll of board paper will have a moisture content different from that of every other roll used in the manufacturing process, and no roll of paper will have a uniform moisture content throughout its length.
Modern paper mills are capable of producing paper, i.e., the liner board and web board used in the corrugation process, which has little or no moisture variation in its width, also known as the cross machine direction. However, the same cannot be said for the variation of the moisture content along the length of the paper, this being known as the longitudinal or machine direction of the paper. Thus, it is important in the corrugated board manufacturing process to measure the machine direction or lengthwise moisture content variation of the board as opposed to its transverse or cross direction moisture content. This problem is amplified due to the fact that corrugated board production line operators are frequently required to obtain paper from several sources, so that in addition to moisture variations between roll to roll from the same paper manufacturer, moisture variations will also exist of either a greater or lesser amplitude between rolls of paper manufactured by other paper manufacturers. This results in a significant inconsistency in paper quality and in machine direction moisture content, which consequently leads to warp in the production of the corrugated board. Moreover, because the problem in controlling moisture content is believed to lie in the machine direction as opposed to the cross machine direction, measurement and control of cross machine moisture content tends not to be economically justifiable in relation to the cost of attempting to control cross machine moisture content in a single zone or through a plurality of zones, as is attempted by Krayenhagen et al., and McGuire et al., U.S. Pat. No. 4,497,027, issued Jan. 29, 1985.
McGuire et al. teaches a method and apparatus for preventing warp in the manufacture of corrugated boards by measuring moisture in the cross machine direction of the liner boards over a plurality of zones transverse to the direction of liner board travel. Thereafter, for each zone of each liner a zonal time averaged moisture value is calculated, and if the zonal time average difference is greater than a predefined average moisture content corrective action is taken by increasing or decreasing the amount of water sprayed onto the single face liner for that particular zone. In addition, if the average moisture content reading for both the single face and double face liners is taken together, and if the combined and averaged moisture content of the top and bottom liner boards is outside of a predetermined range, corrective action is taken by adjusting the pre-heater wrap arm angle for some, or all, of the liner boards.
None of the prior art known to the inventor discloses or illustrates a simple yet efficient method and apparatus for measuring moisture content in the machine direction of corrugated board and for automatically controlling wrap arm movement for the top liner board and bottom liner board wrap arm pre-heater assemblies in response thereto so that the moisture content of the top liner board and bottom liner board falls within a preset moisture content deadband range. Moreover, none of the prior art known to the inventor discloses or teaches a method and apparatus for the automatic control of moisture content of the top liner board separately and apart of the bottom liner board, so that the wrap arms of the pre-heater assemblies for the top liner board and for the bottom liner board are moved independently of each other against separate moisture content setpoints for controlling the moisture content, and thus warping, of the corrugated board as it is manufactured.
Thus, the need exists for an improved method and apparatus for automatically controlling the moisture content of corrugated board during its manufacture, in which the moisture content of the top liner board and bottom liner board of the combined corrugated board is measured in the machine direction of the board, and which allows the moisture content of the top liner board to be measured and adjusted independently of the measurement and adjustment of the moisture content of the bottom liner board in the corrugated board production line.